1. Field of the Invention
The present invention relates to a short arc type discharge lamp used as a light source for light exposure in the field of manufacturing semiconductors and liquid crystals.
2. Description of Related Art
A short arc type discharge lamp has been used as a light source for a light exposure device having a high light collection efficiency in combination with an optical system because the distance between the tip ends of the pair of electrodes disposed inside the arc tube in a manner of facing each other is so short that it is close to a point light source.
Since the electrodes of a short arc type discharge lamp are heated to a high temperature during operation, impure compounds tend to be produced at the tip ends of the electrodes, which reach the highest temperature, if impure gas is mixed in the arc tube. As a result, the evaporation of the electrodes is accelerated. It is believed that impure gas, particularly oxygen and carbon dioxide, tends to accelerate the evaporation of the electrodes because they generate impure compounds, such as oxides and carbides on the tip end of the electrodes.
The accelerated evaporation of the electrodes causes substances evaporated from the electrodes to attach to the inner surface of the arc tube, which leads to the blackening of the arc tube. Moreover, the illumination on the light exposure surface is adversely affected, and there is some possibility that the arc spot may shift because the tip ends of electrodes are deformed due to the evaporation.
In order to absorb and capture impure gas inside the arc tube, it has conventionally been practiced that a getter is attached to an inner support rod used for supporting the electrode. There are several metals that can bring about the getter effect. Among them, the typical metal conventionally used for a short arc type discharge lamp is tantalum. It has been believed that tantalum is the best metal for use as a getter for a small-sized short arc type discharge lamp, which reaches a high temperature inside, in part because the operational temperature that brings about the getter effect is relatively high (700° C. to 1200° C.) and in part because its vapor pressure is low.
Published Unexamined Japanese Patent Application No. H8-153488 and corresponding U.S. Pat. No. 5,712,530 disclose a short arc type discharge lamp having a configuration in which a getter for capturing impurities is attached to a support rod. FIG. 10 is a view showing a schematic configuration of the conventional short arc type discharge lamp disclosed in Published Unexamined Japanese Patent Application No. H8-153488 and corresponding U.S. Pat. No. 5,712,530.
The short arc type discharge lamp as shown in the FIG. 10 has an essentially spherical arc tube 101. Inside the arc tube 101 are disposed a cathode 102 and an anode 103, each of which is supported by a support rod 104 in a manner of facing each other. A metal foil 105 is connected to each support rod 104. The reference numeral 106 is the remnant of a filling tube. A tantalum wire 107 is wound around the support rod 104 and then firmly fixed by spot welding. The temperature of the tantalum wire 107 is in the range of 1500° C. to 1700° C. during operation.
As the size of a lamp increases, a problem referred to as flicker has become obvious in such a short arc type discharge lamp, wherein the illumination fluctuation increases in the range of several milliseconds to a few tens of seconds. After intensively studying this problem, the present inventors found that the concentration of hydrogen inside the arc tube is related to this problem. However, tantalum that has conventionally been used as a getter for a short arc discharge lamp is low in hydrogen absorbing capability, and therefore, cannot absorb enough hydrogen inside the arc tube.
Published Japanese Examined Patent Application No. S57-21835 and corresponding U.S. Pat. No. 3,953,755 disclose the use of yttrium as a metal for a getter in order to remove hydrogen inside an arc tube, yttrium having an excellent hydrogen absorbing capability. FIG. 11 shows the overall structure of the discharge lamp disclosed in the literature. FIG. 12 shows a cross-sectional structure of a getter in the discharge lamp.
The discharge lamp as shown in FIG. 11 is provided with a vessel 111, a pair of electrodes 112, 113, sealing parts 114 and metal foils 115. The reference numerals 116, 117 and 120 are a quartz cylinder, a quartz stick and a hydrogen getter, respectively. As shown in FIG. 12, the hydrogen getter 120 is constituted of a metal cover 123, which is composed of a closed-end cylinder 121 made of such a metal as tantalum and a lid 122, and a getter material 124 made of cylindrical yttrium air-tightly sealed inside the metal cover 123. The inside of the metal cover 123 is air-tightly sealed by resistance welding a shoulder 121a of the closed-end cylinder 121 together with the lid 122. As shown in the drawing, the hydrogen getter is fixed using the quartz cylinder 116 and is fixed to the vessel 111 by welding the other end of the quartz stick 117 provided on the quartz cylinder 116 to the vessel 111. Hydrogen inside the vessel 111 infiltrates into the inner portion of the metal cover 123 made of tantalum or the like, which allows hydrogen to pass through the metal cover 123 to be absorbed by the getter material 124. The hydrogen getter 120 described in these documents can absorb hydrogen without reacting with other materials in the discharge space because the getter material 124 is sealed air-tight inside the metal cover 123.
There is some possibility, however, that the attachment of the hydrogen getter 120 to the vessel 111 as described above may cause a decline in illumination or the fracturing of the vessel because the hydrogen getter 120 may react with silica, a constituent of the vessel 111.